Various embodiments of a method of forming a solder joint are described herein. In particular, the embodiments described herein relate to a method of forming a solder joint with improved process control and superior solder joint reliability.
Solder is frequently used in the production of electronic components to join integrated circuit modules or chip carriers to circuit cards or circuit boards. For example, solder may be used to connect conductive metal pins from a module to the conductive circuit lines of a circuit card. It is known to apply solder to the components in the form of a solder preform or a solder paste. A solder preform is a solid composition of solder or braze material fabricated to the shape and dimensions required to contact the desired locations of each of the components to be joined. The preform is placed in proper position and subsequently heated to cause the solder to flow, or “reflow,” and physically join the components.
Solder paste is a composition of a solder powder in one or more liquid solvents or binders. The paste is screened on to the components, dried, and heated to reflow the solder and join the two components. For both preforms and pastes, a liquid flux is typically used to deoxidize the metal surfaces of the components to cause them to accept the solder.
The use of a conventional liquid flux results in flux residues left behind on the surface of the components after soldering. For example, a common flux such as waterwhite rosin leaves a metal salt residue abietate formed when the abietic acid in the rosin reacts with oxides on the metal surfaces of the components. Where the residue contacts the metal surfaces of the components, it will cause detrimental galvanic corrosion upon the passage of electrical current in normal use. Thus, the residue must be removed from the components after soldering and before electrical use.
Another problem associated with that of soldering electronic components is that of precision. To assure that proper electrical connections, electrical components must often be joined according to tight dimensional tolerances. Solder preforms are inherently difficult to use in such applications because precision is limited by the accuracy by which the preform is placed upon the components, and fixtures must be used to hold the electrical components, circuit modules, or chip carriers to circuit cards or circuit boards during the solder reflow process.
Micro electro mechanical systems (MEMS) are a class of systems that are physically small, having some features or clearances with sizes in the micrometer range or smaller (i.e., smaller than about 10 microns). These systems have both electrical and mechanical components. The term “micro machining” is commonly understood to mean the production of three-dimensional structures and moving parts of MEMS devices. MEMS originally used modified integrated circuit (e.g., computer chip) fabrication techniques (such as chemical etching) and materials (such as silicon semiconductor material) to micro machine these very small mechanical devices. Today there are many more micro machining techniques and materials available. The term “MEMS device” as may be used in this application is defined as a device that includes a micro machined component having some features or clearances with sizes in the micrometer range, or smaller (i.e., smaller than about 10 microns). It should be noted that if components other than the micro machined component are included in the MEMS device, these other components may be micro machined components or standard sized (i.e., larger) components. Similarly, the term “microvalve” as may be used in this application means a valve having features or clearances with sizes in the micrometer range, or smaller (i.e., smaller than about 10 microns) and thus by definition is at least partially formed by micro machining. The term “microvalve device” as may be used herein means a device that includes a microvalve, and that may include other components. It should be noted that if components other than a microvalve are included in the microvalve device, these other components may be micro machined components or standard sized (i.e., larger) components. The term “MEMS package” as used herein should be understood to mean a device, which includes a micromachined component and may include other components that may be micromachined components or standard sized components. A “MEMS fluidic package” should be understood to be a MEMS package including a fluid passageway. A “MEMS electrofluidic package” as used herein should be understood to be a MEMS package including a fluid passageway and an electrically active component that may be a micromachined component. A “MEMS package platform” as used herein should be understood to be an interface component or assembly of components upon which a MEMS device may be mounted and by means of which the MEMS device can be interfaced with an external system.
Many MEMS devices may be made of multiple layers (or substrates) of material, which may be micromachined to form components of the MEMS device prior to assembly of the multiple layers into a completed MEMS device. For example, such a MEMS device may be manufactured using suitable MEMS fabrication techniques, such as the fabrication techniques disclosed in U.S. Pat. No. 6,761,420, the disclosures of which are incorporated herein by reference; U.S. Pat. No. 7,367,359, the disclosures of which are incorporated herein by reference; Klassen, E. H. et al. (1995). “Silicon Fusion Bonding and Deep Reactive Ion Etching: A New Technology for Microstructures,” Proc. Transducers 95 Stockholm Sweden, pp. 556-559, the disclosures of which are incorporated herein by reference; and Petersen, K. E. et al. (Jun. 1991). “Surface Micromachined Structures Fabricated with Silicon Fusion Bonding”, “Proceedings, Transducers” 91, pp. 397-399, the disclosures of which are incorporated herein by reference.
Flux and/or flux residue may undesirably coat the internal moving components of the MEMS device when a MEMS device is attached to a substrate using known soldering processes.
The above notwithstanding, there remains a need in the art for an improved method of forming a solder joint.